A method of diagnosing a fault such as overheat abnormality of an oil-immersed electric appliance such as a transformer without turning off the appliance with a type or a concentration of a gas component in insulating oil or a concentration ratio between a plurality of gas components being defined as an indicator has been known (for example, NPL 1 (Z. Wang, X. Wang, X. Yi and S. Li, “Gas Generation in Natural Ester and Mineral Oil Under Partial Discharge and Sparking Faults,” IEEE Electrical Insulation Magazine, Vol. 29, No. 5, pp. 62-70, 2013) and NPL 2 (Denki Kyodo Kenkyu, Vol. 54, No. 5 (1), Aburairi Hen-atsuki no Hoshu Kanri, Electric Technology Research Association, February 1999)).
Many works associated with such a fault diagnosis method for an oil-immersed transformer in which mineral oil is employed as insulating oil through analysis of gas in oil have been built up and guidelines for diagnosis of a fault have also been proposed domestically and abroad.
For example, in an oil-immersed electric appliance (such as a transformer) to be mounted on a vehicle (such as a rail vehicle), with safety being focused on, non-mineral oil based insulating oil (such as silicone oil) higher in flash point and higher in safety than mineral oil is sometimes employed. In recent years, ester oil which is excellent not only in safety but also in biodegradability and low in environmental loads has increasingly been applied (for example, PTL 1 (Japanese Patent Laying-Open No. 2013-131511)).
Non-mineral oil based insulating oil such as silicone oil or ester oil, however, is different in composition from mineral oil, and therefore it is different from mineral oil in type and ratio of concentration of gas components generated at the time of occurrence of such a fault as discharging and overheating.
PTL 2 (Japanese Patent No. 5872127) discloses a method of diagnosing a fault (estimating an overheating temperature) based on a concentration ratio between gas components in silicone oil in an oil-immersed electric appliance in which silicone oil is employed as insulating oil.